debloat jmp_buf in _GNU_SOURCE mode

i originally made it the same size as the bloated GNU version, which
contains space for saved signal mask, but this makes some structures
containing jmp_buf become much larger for no benefit. we will never
use the signal mask field with plain setjmp; sigsetjmp serves that
purpose.

include/setjmp.h

@@ -12,9 +12,6 @@ extern "C" {
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) \
  || defined(_BSD_SOURCE)
 typedef unsigned long sigjmp_buf[(128+sizeof(jmp_buf))/sizeof(long)];
-#ifdef _GNU_SOURCE
-#define jmp_buf sigjmp_buf
-#endif
 int sigsetjmp (sigjmp_buf, int);
 void siglongjmp (sigjmp_buf, int);
 #endif